A loop antenna may be utilized in a variety of systems, including, for example, EAS and RFID systems. An excitation source may provide current to, or induce current in, the loop antenna at a particular excitation frequency. The excitation source may include a tuning capacitor, where the value of the capacitor is used to set the excitation frequency. Current flowing along the length of the loop antenna generates an electromagnetic field in proportion to the current flow. If the current flow through the antenna exhibits wide variances, then the resulting electromagnetic field will exhibit corresponding variances. This leads to degradation in performance of systems utilizing such antennas.
To avoid this degradation, conventional loop antennas have been constructed to be “electrically small.” An electrically small loop antenna is an antenna wherein its physical length is short relative to its operating wavelength, i.e. typically not more than 1/10th of the wavelength. The operating wavelength λ of the field generated by the antenna in free space is given equation (1), where c is the speed of light, and f is equal to the excitation frequency provided by the excitation source.λ=c/f  (1)
As the physical length of the loop antenna becomes greater than 1/10th of the wavelength λ, appreciable current variances are manifested along the length of the loop.6 Simulations and tests on prototype antennas of differing lengths have been conducted to determine the extent of current variance along the length of loop antennas. For a wavelength of 22.12 meters, corresponding to an excitation frequency of 13.56 MHz, loop antennas having lengths of 2.0 meters, 4.0 meters, and 8.0 meters have been tested. In general, the current variation along the length of the antenna increased substantially as the length of the loop antenna was increased beyond 1/10th of the wavelength.
For instance, for a loop antenna length of 2.0 meters, a 0.5 m×0.5 m square loop antenna operating at an excitation frequency of 13.56 MHz was simulated and tested. Of course, the 2.0 meter length is slightly less than 1/10th of the 22.12 meter wavelength. The simulation was performed using the Expert MININEC Series by EM Scientific, Inc. The Expert MININEC Series is a software tool that utilizes method of moments to solve for currents and electromagnetic fields for electrically thin wires. The simulation revealed that the current around the 2.0 meter loop antenna increased to a maximum level of only about 4% higher than a minimum level. An experimental measurement of the a prototype 0.5 m×0.5 m loop antenna using a Pearson RF current probe also indicated a 4% variation of current magnitude around the loop antenna.
For the longer loop antenna of 4.0 meters, a 1.0 m×1.0 m square loop antenna at 13.56 MHz was simulated using the Expert MININEC Series program. The 4.0 meter length is almost ⅕th of the 22.12 meter wavelength. The simulation revealed a current magnitude variation of about 17% comparing a maximum current level to a minimum current level around the loop antenna. An experimental measurement of a prototype 1.0 m×1.0 m loop antenna using a Pearson RF current probe indicated a 30% variation of current magnitude around the loop antenna.
Yet an even longer loop antenna of 8.0 meters, configured as a 2.0 m×2.0 m square loop antenna at 13.56 MHz, was simulated using the Expert MININEC Series program. The 8.0 meter length is almost ⅖th of the 22.12 meter wavelength. The simulation revealed a current magnitude variation of as much as 235% comparing a maximum current level to a minimum current level around the antenna. Whenever such large asymmetries exist in the antenna current distribution, as indicated by both the simulated and experimental results, the resulting electromagnetic fields are also asymmetrical. Again, this leads to degradation in performance of systems utilizing such antennas, and in particular is highly undesirable for field-canceling antennas which rely on even field distribution for canceling affects.
It is clear, therefore, that increasing the loop length of conventional antennas to degree appreciably greater than 1/10th of the operating wavelength causes increasingly larger current variations around the antenna. These current variations lead to degradation in performance of systems utilizing such antennas. In systems wherein the excitation frequency is fixed, e.g. in some EAS and RFID systems, current variations associated with exceeding an antenna length of 1/10th of the operating wavelength place a practical limit on the physical length of the loop antenna. Limited antenna length limits the effective range of such antennas. Also, where such antennas are provided in nested configurations to achieve far-field canceling benefits, the length of the inner loop antenna is constrained by the limited length of the outer loop antenna. As such, the range of an inner loop antenna in a nested loop configuration is also limited.
Accordingly, there is a need in the art for a loop antenna configuration wherein the antenna length may be greater than 1/10th of the operating wavelength without causing unacceptable current variance about the antenna length.